The invention relates to an internal combustion engine having a turbocharger with variable turbine geometry.
In supercharged diesel internal combustion engines used in commercial vehicles, it is known to furnish the turbines of the turbochargers with variable geometry to adjustably set the effective turbine cross section. In this way, it is possible to achieve desired levels of exhaust backpressure in the segment between the cylinders and the turbocharger, as a function of the operating state of the internal combustion engine. As a result of the variable geometry the power output of the turbine and the work done by the compressor can be adjusted as needed. A supercharged internal combustion engine having variable turbine geometry is known, for example, from German Patent Application 195 43 190 A1.
To achieve an engine braking effect during braking operation of the internal combustion engine, the inlet guide baffle of the turbine is placed in a blocking position in which the turbine cross section is significantly reduced, so that a high exhaust backpressure is then built up, opposing the expulsion through exhaust valves into the exhaust-gas manifold of the air being compressed in the cylinder. In the engine braking operation, the piston in the compression and exhaust stroke must perform compression work against the high overpressure in the exhaust manifold, as a result of which a powerful braking effect is achieved.
In addition, it is known from the published European Patent Application 620 365 A1, that to limit nitrous oxide emissions the exhaust gas of supercharged internal combustion engines should be routed back into the intake line, when the engine produces power. In accordance with European Patent Application 620 365 A1, a second smaller turbocharger is attached parallel to a main turbocharger, and is employed exclusively for transferring the exhaust gas. Exhaust gas is extracted downstream of the turbine of the main turbocharger, is compressed in the compressor of the smaller turbocharger, and is fed again into the intake pipe of the engine. It is disadvantageous that the exhaust gas extracted downstream of the turbine is already expanded, so that only a very small energy potential is still available for driving the smaller turbine of the supercharger employed for conveying the exhaust gas. A further disadvantage lies in the cumbersome design of the device, encompassing two turbochargers, as a result of which construction costs, space requirements, and factory costs are increased.
The invention is a supercharged internal combustion engine having variable turbine geometry in such a manner that, using simple means and at small cost, an exhaust-gas recirculation system can be realized that avoids overtaxing the engine in extreme operating ranges.
According to a first inventive embodiment, the range of motion of the variable turbine geometry is limited by a movable limit stop, which, in the event that the exhaust gas recirculation is opened or is about to be opened, limits the adjustment range of the variable turbine geometry to a definite, preselected value. This embodiment has the advantage that no cumbersome closed-loop control is necessary for adjusting the turbine geometry. Rather, in a manner that is preferably controlled by open-loop and not by closed-loop, the turbine geometry is adjusted between the open position and a blocking position, until the limit stop is reached. The limit stop is moved from its release position, outside a control path defining the range of motion of the turbine geometry, to the limiting position in which the adjustment range of motion of the turbine geometry is limited. By dispensing with the closed-loop control in adjusting the turbine geometry, the control-engineering problems that can arise at small rotational speeds through the strong dependence of the exhaust-gas recirculation rate on the position of the turbine geometry are avoided.
To determine the releasing criterion for moving the limit stop from the release position to the limiting position, the conditions for the start of exhaust-gas recirculation are considered. If the conditions are met, then the limit stop is placed in the limiting position. Conversely, in the event that the conditions for the exhaust-gas recirculation are not satisfied, the limit stop is placed back into the release position and the recirculation of the exhaust gas from the exhaust-gas line into the intake line is interrupted. In the release position of the limit stop, the turbine geometry can move along the entire control path unhindered. The exhaust-gas recirculation and limit stop are actuated by actuating signals generated in a closed-loop and open-loop control device.
In order to create the pressure conditions necessary for exhaust-gas recirculation, which include an exhaust-gas backpressure that is higher than the boost pressure, it is advisable to initially position the limit stop in the limiting position, so that the turbine entry cross section is reduced and the exhaust-gas backpressure rises. As soon as the exhaust-gas back pressure exceeds the boost pressure, the exhaust-gas recirculation system is opened.
A limiting piston can be conveniently used as an adjustable limit stop which can be actuatable mechanically, electrically, hydraulically, or pneumatically.
The limit stop is used advantageously for limiting the movement range along the control path of a control element that acts upon the turbine geometry and that moves the latter between the blocking position and the open position. This indirect limiting of the range of motion of the turbine geometry has the advantage that the effective turbine cross section is neither hindered nor infringed upon by the limit stop extending into the control path. If appropriate, however, it can also be advantageous to directly limit the position of the variable turbine geometry by the limit stop.
When the exhaust-gas recirculation system is activated, it is preferable that the limit stop adopt only one single, defined limiting position independent of the rotational speed, the limiting position representing an optimal value for a specific rotational speed. Depending on the engine load, this usually results in recirculation rates of about 5% to about 25%, that are adjusted for the specific rotational speed. The recirculation rate is the ratio of the re-circulated exhaust-gas flow to the entire mass flow fed to the cylinders, composed of fresh air and exhaust gas. As the rotational speed rises, the exhaust-gas recirculation rate also rises, assuming the limiting position of the limit stop has not been altered. Since the internal combustion engine can tolerate higher exhaust-gas recirculation rates as the rotational speed and boost pressure rise, the increasing exhaust-gas concentration is nevertheless tolerable, so that even if the limit stop has only one limiting position resulting in the turbine geometry having only one position for all rotational speed ranges, satisfactory results can be achieved.
For more precise adjustment of the quantity of re-circulated exhaust gas, it can be advantageous to assign a different limiting position of the limit stop to different rotational speeds. In this manner, an optimized exhaust gas recirculation rate is attained, and an undesirably high exhaust-gas concentration is avoided. It is advantageous to assign the limiting positions to the rotational speeds in discrete steps, a limited number of limiting steps being assigned to an equal number of rotational speed ranges. In a preferred embodiment, provision is made for two limiting positions corresponding to two different rotational speed ranges. This number of limiting positions is usually adequate for a sufficiently precise exhaust gas recirculation (EGR) rate.
The discrete, two-step assignment of two limiting positions to two rotational speed ranges is advantageously realized with the assistance of two different limit stops, which can be two limiting pistons, both of which are able to enter into the control path of the turbine geometry at different limiting positions. The given limiting position is set by activating the respective limiting piston as a function of the rotational speed range detected by the closed-loop and open-loop control device, in the event that active exhaust-gas recirculation occurs.
In a further advantageous embodiment, provision is made for only one single limit stop, which adopts various limiting positions as a function of the rotational speed resulting in a space-saving design. Moreover, in addition to discrete adjustment, a continuous adjustment of the limit stop is also possible, so that the greatest possible precision can be achieved.
In accordance with a second inventive embodiment, an exhaust-gas intercooler is disposed in the recirculation line of the exhaust-gas recirculation system. Charge air is used as a coolant in the exhaust-gas intercooler. In this way, when turbochargers having variable turbine geometries are employed, it is possible to use the excess air flowing in the intake line for cooling the hot, re-circulated exhaust gas. The boost pressure of the engine is thus limited, as a result of which it is assured that peak pressure limits are not exceeded, above which damage can occur to the engine. The heat removed from the charge air in the coolant circuit can be expelled into the surrounding atmosphere. In contrast to conventional exhaust-gas recirculation water coolers, no heat dissipation via the cooling system of the engine is necessary, so that fuel is saved due to the smaller power requirements of fans and water pumps.
In an advantageous embodiment, provision is made at the compressor entry for an air mass sensor, which measures the air mass flow entering into the compressor in order to establish the combustion air ratio. If an excess of air is determined, then the excess air component is expelled via the exhaust-gas cooler.
In further embodiments, in place of an air mass sensor at the entry of the compressor, it is possible also to use a pressure sensor in the charge air pipe or a lambda sensor in the exhaust-gas pipe to determine the combustion air ratio.